Saturday, January 8, 2011

Seeing the Light!

It started with an e-mail Friday. An author of an upcoming astronomy text found me on the Internet from a picture of the Hyades star cluster I had taken a decade or more ago for McDonald Observatory. I had made a prism out of flint glass, and experimented some with objective prism spectroscopy. After Googling it, mine was about the only image that popped up, even though it was pretty lousy, from way back in the film days. Anyway, he was asking if it was copyrighted anywhere, or if he could use it for his book. The old picture is shown here.



My thinking was that it could be greatly improved with modern cameras and detectors, so after digging the prism out of my "storeroom", I modified it for my digital camera and telephoto lens. After a test in the backyard last night on Sirius, Melinda and I headed up to Geology Vista in the mountains north of town in search of darkish skies (moon was out, so didn't need a long trip!). After setting up the tracking mount (G-11), The Hyades star cluster was centered, the proper zoom setting located (about 150mm), and the exposure fine tuned (45 seconds, ISO 800), and technique practiced (driven north at slowest rate during the exposure to widen spectra). The "V" shape of the Hyades cluster is evident, though the orientation is rotated 90 degrees from the old image atop the page. The bright star is Aldebaran, a K5 giant.

The results I thought were quite spectacular! Not only were the stars spectral colors nicely displayed with the Bayer-matrix detector of the Canon camera (XSi in this case), but unlike the old film image, the spectral absorption lines were readily visible in nearly all the stars. The dark lines silhouetted on the spectra indicate the chemicals and their abundances in the distant stellar atmospheres, and can be used to determine temperatures of the stars as well. Shown here is a blow-up of the above image, zeroing in on Theta 1 (above) and Theta 2, a multiple star and members of the cluster. The upper star is a K0III, a cool giant star where many absorption lines show the makeup of it's atmosphere. The lower is an A7III, whose hotter temperatures and few strong lines show mostly hydrogen in it's atmosphere.

Objective prism spectroscopy in combination with wide-field Schmidt telescopes was used extensively for surveys of stellar spectral types, and more recently just a couple decades ago for galaxy surveys. The ability to see the spectra of many objects at once was a very efficient way to work. Now days it is used less, but it is a fun and new way to see the sky in a new light!

5 comments:

Anonymous said...

NIce work, a great story, and kudos to Melinda for her encouragement.

soooo... is he going to publish the original or a newer version?

Anonymous said...

Very cool Dean! Keith

Rick Scott said...

Very nice. Excellent example of what can be done with simple equipment.

David A. Harvey said...

Very Nice Dean! Congrats on getting published too. Where did you get the objective prism? Ever try this with diffraction Grating? Advantages/disadvantages of one over the other?

Dean said...

Hi Dave-
Well, it's not published yet - haven't heard back from the author... Being an optician, I can make a prism, but not a diffration grating! Originally I threw it together in a night or two to get a spectrum of Hyakutake, so that is how old it is.

I believe you get better wavelength dispersion characteristics from a grating - the prism doesn't disperse as well in the red, so often appears brighter when scrunched together. Though ALL the light from a prism is dispersed, no multiple orders like a grating.

Let me know if you would like to borrow it!

-Dean